In
the UKaccounting is
explaining oneself or one's actions ("to give an account" or
"accountability" in the U.S.A.),accountancy is
the profession.

Action Man

the
action figure toy sold in the US
as G.I. Joe.

advert

advertisement (US
and UK
also: ad, commercial (on TV)).

agony aunt

the
author of an agony column – a magazine or newspaper column
advising on readers' personal problems. The image presented was originally that
of an older woman providing comforting advice and maternal wisdom, hence the
name "aunt". Better known to most Americans as a "Dear
Abby" column or advice column. Similarly, agony uncle.

air marshal

an
Air Force officer of high rank (US: general)

all change

announcement
on train or bus on approaching the last stop (US: All out)

amidst

Both
"amidst" and "amid" are common in the UK, whereas in the US "amidst" is often
considered old-fashioned.

amongst

Still
in wide usage in the UK,
with the alternative among also used. Amongst is considered archaic
in US
usage, but is still occasionally used.

anorak

In
the US,
a jacket with a fur-lined hood is generally called a "parka,"
technical differences between the two garments notwithstanding. As a slang term
for someone with an obsessive interest in a niche subject (most famously,trainspotters),
"anorak" is also a Britishism (no direct US analog, but similar to the
Japanese "otaku," which has migrated into US English).

(old-fashioned)
school for juvenile delinquents; reform school. Such institutions have not
been referred to officially as "approved schools" since 1969. Juvenile
delinquents, depending on their age and level of malfeasance, may now be sent
to Secure Training Centres (for ages 15 to 18) or YOIs (Young Offender
Institutions – a prison for offenders aged between 18 and 21). (US: juvenile
detention center, JDC, juvenile hall, (slang) juvie.)

argy-bargy

(informal)
a disagreement ranging from a verbal dispute to pushing-and-shoving
or outright fighting.

arse

buttocks,
backside or anus, depending on context (US equivalent: ass);
to be arsed: to be bothered to do something, most commonly as a
negative or conditional (e.g. I can't be arsed, if/when I
can be arsed).